1. Field of the invention
The present invention relates to a drive and support structure of an objective lens, and more specifically to a drive and support structure of an objective lens of a pickup unit of a system for optically reading recorded information.
2. Description of Background Information
In systems for optically reading recorded information, an operation of an optical pickup unit is such that it focuses a reading laser beam on a recording track of a recording medium and it receives a reflected laser beam from the recording medium. The recorded information is recovered from the recording medium in terms of the change in the reflection of the irradiated reading laser beam. In the operation of the optical pick up unit, it is necessary that the reading laser beam is correctly focused on the recording track irrespectively of the fluctuation of the level of the recording surface relative to the pickup unit, for instance, caused by a curvature of the recording medium. Therefore, the optical pickup unit is provided with a drive and support structure of the objective lens which moves the objective lens in a direction perpendicular to the recording surface within a relatively small amplitude.
The drive and support structures of an objective lens presently in use are constructed such that a pole piece of a magnetic circuit is inserted into a space between the objective lens and a bobbin of a focusing coil fixed to the objective lens and having a diameter greater than that of the objective lens.
However, this type of construction is disadvantageous in that the total weight of the driven parts, i.e. the objective lens, the focusing coil, and the bobbin becomes relatively large, which in turn adversely affects the response charcteristic of the focus servo system. Further, it is also disadvantageous that the diameter and the height of the drive and support structure are large. This problem of the large size of the drive and support structure of the lens has been a major cause which prevents the total reduction of the size of the information playback system. In addition, there is also a disadvantage that the assembling of the drive and support structure is rather difficult due to the complicated structure.